The present invention generally relates a pressure-responsive switch assembly including a first spring and a second spring designed such that, when a microswitch is turned on or off, the first spring alone counteracts a working pressure and, under other conditions, the action of the first spring is suppressed by the second spring. More particularly, the present invention is concerned with such a switch assembly having an improved device for anchoring the second spring.
A pressure switch of such type is disclosed in Japanese Utility Model Publication No. 37-14418 (1962). One preferred embodiment of the prior art pressure-responsive switch of such type comprises, as shown in the attached drawings (FIGS. 1 and 2), a bellows (12) which is secured around an opening provided in the wall of a casing (1) and is connected to a pressure source. The free end of the bellows (12) is engaged by a generally L-shaped actuating member or plate (8) pivotably connected to the casing (1) to turn about a fulcrum (15) because of the action of the bellows. A first or compression spring (6) has one end thereof abutted against the actuating plate (8) at a vertical arm portion (8b) in opposing relation to the pressure admitted in the bellows (12). The first spring is secured to one end to the side wall of the casing (1) through an adjusting screw (3) and extends and has horizontally a point contact member at its other end which contacts the vertical arm portion of the L-shaped actuating member or plate. The adjusting screw (3) is rotatably secured to the casing wall and can adjust the resilient force of the first spring (6). The actuating plate has a lateral arm portion (8b) which engages with a contact member (11) of a microswitch (10) to open and close the switch. The vertical arm position (8b) of the actuating plate is connected to one end of a second or tension spring (5) by means of a connecting plate (14) for distributing pressure differential. The end portion of the vertical arm portion (8b) is loosely engaged with an opening provided in the connecting plate (14) which has another opening holding a hook end portion of the second spring (5). The movement of the plate (14) is limited to a predetermined range by a stopper (7). The second spring (5) is also secured to the side wall of the casing (1) and laterally extends parallel to the first spring (6). The second spring (5) resilient force may be adjusted by means of a screw (2) also rotatably secured to the side wall of the casing (1). FIG. 1 shows a situation of the switch assembly in which the bellows has contracted due to the drop or lowering of the pressure maintaining the microswitch (10) in the off-state. FIG. 2 illustrates the on-state of the microswitch derived from an increase in the pressure.
When the pressure acting in the bellows increases in the situation depicted in FIG. 1, the bellows expands causing the actuating plate (8) to move clockwise about the fulcrum (15) so that the contact member (11) of the microswitch is depressed by the arm portion (8a) of the actuating plate. Meanwhile, the connecting plate (14) anchored to the other arm portion (8b) of the actuating plate is pulled to the right as viewed in the drawing under the action of the second spring (5) until its movement is limited by the stopper (7). The second spring thus stopped can not impart its force to the actuating plate so that the pressure inside the bellows is opposed by the force of the first spring (6) alone. When a predetermined pressure selected by the screw (3) is reached, the contact member (11) is depressed thus turning the microswitch on as best shown in FIG. 2. Conversely, when the pressure inside the bellows drops or falls under the conditions depicted in FIG. 2, the first spring (6) urges the actuating plate (8) counterclockwise about the fulcrum (15). However, the actuating lever has the movement of its arm portion (8b) suppressed by the second spring (5) anchored to the plate (14) and, hence, the pressure acting in the bellows is counteracted by the difference between the force of the first spring and that of the second spring. In other words, the composite force of the pressure inside the bellows and the force of the second spring essentially counteracts the force of the first spring. Accordingly, the situation of FIG. 1 in which the microswitch (10) is switched off is not restored unless the pressure inside the bellows undergoes an extra drop or fall corresponding to the force of the second spring. The resilient force of the second spring (5) is adjustable by the screw (2).